In the audio power amplifier field, the customer demands lower cost solutions without sacrificing performance. Class-D audio amplifiers have great efficiency and eliminate heat sinks, significantly reducing overall system cost and size. Class-D audio amplifiers are becoming quite popular because of the above advantages. Generally, Class-D audio amplifiers are classified into two types according the two different input types: analog and digital.
The analog Class-D amplifiers usually use negative feedback structure, which can improve the Total Harmonic Distortion (THD) and Power Supply Rejection Ratio (PSRR).
In the prior art, most the digital input Class-D amplifiers use open-loop structures, which have an output duty cycle that is equal to the input duty cycle. The gain of open-loop digital Class-D is dependent on the power supply voltage Vcc, and the input and output can reach full scale range at different Vcc. However, the open-loop digital Class-D amplifier has poor PSRR, since the duty cycle does not change to compensate for ripple in the power supply voltage.
Nowadays, increasingly applied digital audio signal processing makes it more convenient to use a digital input audio power amplifier rather than an analog input. The digital input Class-D amplifier with close-loop structure is therefore proposed for better performance of THD and PSRR. Audio digital power amplifier circuits usually have multiple power supplies, the input PWM signal is logic level, and the output power driver stage has a wide operative power supply range. The voltage gain of the closed-loop digital Class-D is determined by the feedback loop, when the amplifier circuit is determined, the gain is fixed. The full scale input duty cycle is about from 0 to 100%, but the output duty cycle is different at different output power supply voltage Vcc even when the input duty cycles are the same, since the voltage gain of the negative feedback loop is fixed. The drawback is that the closed-loop structure with fixed gain cannot reach linear full scale input and output duty cycle range at different power supply voltage. The output will be clipped early at low Vcc, and cannot reach full scale duty cycle at high Vcc. Thus, the audio signal carried by the duty cycle of the input PWM signal will be distorted early at low Vcc, and the output power is limited by the gain at high Vcc.
There is a need in the art to realize the full scale input and output range of a digital Class-D circuit with wide output power supply voltage range.